Known in the state of the art are so called oscillatory forks, which serve for monitoring a limit level of a liquid in a container, or for density or viscosity measurement. The mechanically oscillatable unit in the form of the oscillatory fork is excited by an electromechanical transducer unit to execute resonant mechanical oscillations. The electromechanical transducer unit receives the oscillations of the oscillatable unit and transduces them into an electrical, received signal dependent on the oscillation characteristics. Based on the electrical, received signal, the process variable is determinable. For fill level measurement, it is, for example, detected whether the oscillation frequency in the case of resonant exciting lies below or above a predetermined limit frequency. If the oscillation frequency exceeds the limit frequency, the oscillatable unit is oscillating in air; if the oscillation frequency subceeds, or falls beneath, the limit frequency, the oscillatable unit is covered with medium.
There are essentially two types of transducer units known: piezoelectric and inductive. Piezoelectric transducer units have either one piezoelement (e.g. German Application DE 3931453 C1) for exciting and receiving the oscillations or, separately, at least one exciter piezo and one receiver piezo (e.g. German Application DE 19720519 A1). Inductive transducer units have, as a rule, one coil for exciting the oscillations and another coil for receiving the oscillations. It is, however, also known, to use a coil for exciting the oscillations and a piezoelectric element for receiving the oscillations (e.g. German Application DE 4320411 C2).
In so-called one-piezo technology, the same piezoelement serves as a transmitter and a receiver of the mechanical oscillations of the oscillatable unit. Serving for supplying power to the piezoelement is, as a rule, an electrical alternating voltage having a rectangular waveform. In supplying a piezoelement with such a rectangular signal, the piezo capacitance undergoes reverse poling at each edge of the rectangular signal. This leads to charging and discharging, electrical currents. Additionally flowing is an electrical current corresponding to the mechanical movements. For evaluating the mechanical oscillations, the resulting electrical current is converted via a resistor into a voltage. The charging and discharging, electrical currents represent undesired disturbance signals in the evaluation.
Somewhat the same holds for one coil. Because of the strong coupling between transmitted signal and received signal, a wanted signal attributable to the oscillations of the oscillatable unit is only difficultly extractable based on the received signal. Therefore, in the case of an inductive drive, often the received signal is registered, not by means of a coil, but, instead, by means of a piezoelectric transducer element. The force coupling with the oscillatable unit required for piezoelements leads, however, for example, in the case of strong temperature fluctuations, to high forces on the piezoelectric transducer element.